For example, a fixed type constant velocity universal joint can be taken as an example of a constant velocity universal joint used as means for transmitting a rotational force from an engine to wheels of an automobile at a constant velocity. The fixed type constant velocity universal joint has a structure in which two shafts on a driving side and a driven side are coupled to each other and rotational torque can be transmitted at a constant velocity even when the two shafts form an operating angle. Generally, a Birfield type (BJ) constant velocity universal joint and an undercut-free type (UJ) constant velocity universal joint have been widely known as the above-mentioned fixed type constant velocity universal joint.
Further, as illustrated in FIG. 6, the fixed type constant velocity universal joint of the Birfield type (BJ) includes: an outer race 3 having an inner surface 1 in which a plurality of track grooves 2 are equiangularly formed along an axial direction and serving as an outer joint member; an inner race 6 having an outer surface 4 in which a plurality of track grooves 5 are equiangularly formed in pairs with the track grooves 2 of the outer race 3 along the axial direction and serving as an inner joint member; a plurality of balls 7 interposed between the track grooves 2 of the outer race 3 and the track grooves 5 of the inner race 6, for transmitting torque; and a cage 8 interposed between the inner surface 1 of the outer race 3 and the outer surface 4 of the inner race 6, for retaining the balls 7. In the cage 8, a plurality of window portions 9 for housing the balls 7 are arranged along a circumferential direction.
On opening edges (side edges) of each of the track grooves 2 of the outer race 3 and opening edges (side edges) of each of the track grooves 5 of the inner race 6, in order to avoid stress concentration on both the side edges thereof, chamfers 10, 10, 11, and 11 are provided as illustrated in FIGS. 7 and 8.
In some conventional cases, the chamfers are finished so as to have a round shape (Patent Literatures 1 to 3). By finishing of each of the chamfers into a round shape as just described, stress concentration upon application of high torque (upon input of excessive torque from a vehicle) is easily reduced. Further, the round-shaped chamfers are designed to prevent the edges from being chipped when the balls are pressed against the track grooves and climb onto track edges (track-groove side edges) upon the application of high torque. As a result, shortening of a service life is prevented.
Incidentally, as illustrated in FIG. 7, on an opening side of the outer race 3, there is provided an inlet tapered portion 12 functioning as an angle-limitation stopper so that a shaft does not form more than a certain angle when forming an angle. Normally, a track-groove corresponding edge portion 12a on the inlet tapered portion 12 (edge portion on an axial end portion of each of the track grooves) is formed as a sharp edge. However, in order to reduce stress concentration at a high angle, the track-groove corresponding edge portion 12a is chamfered by a machining process in some cases. Further, as illustrated in FIG. 8, an axial edge 13 of each of the track grooves 5 of the inner race 6 is formed in a shape of a sharp edge portion.